In the fabrication of semiconductor devices from a silicon wafer, a variety of semiconductor processing equipment and tools are utilized. One of these processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer, which are frequently used in memory devices. The planarization process is important since it enables the subsequent use of a high-resolution lithographic process to fabricate the next-level circuit. The accuracy of a high resolution lithographic process can be achieved only when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of semiconductor devices.
A global planarization process can be carried out by a technique known as chemical mechanical polishing, or CMP. The process has been widely used on ILD or IMD layers in fabricating modern semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically-actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum.
A polishing pad used on a rotating platen is typically constructed in two layers overlying a platen, with a resilient layer as an outer layer of the pad. The layers are typically made of a polymeric material such as polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is typically made several times the diameter of a wafer in a conventional rotary CMP, while the wafer is kept off-center on the pad in order to prevent polishing of a non-planar surface onto the wafer. The wafer itself is also rotated during the polishing process to prevent polishing of a tapered profile onto the wafer surface. The axis of rotation of the wafer and the axis of rotation of the pad are deliberately not collinear; however, the two axes must be parallel. It is known that uniformity in wafer polishing by a CMP process is a function of pressure, velocity and concentration of the slurry used.
A CMP process is frequently used in the planarization of an ILD or IMD layer on a semiconductor device. Such layers are typically formed of a dielectric material. A most popular dielectric material for such usage is silicon oxide. In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed is normally between about 5000 A and about 10,000 A. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window-etching or plug-formation difficulties. The CMP process has also been applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.
Important components used in CMP processes include an automated rotating polishing platen and a wafer holder, which both exert a pressure on the wafer and rotate the wafer independently of the platen. The polishing or removal of surface layers is accomplished by a polishing slurry consisting mainly of colloidal silica suspended in deionixed water or KOH solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure uniform wetting of the polishing pad and proper delivery and recovery of the slurry. For a high-volume wafer fabrication process, automated wafer loading/unloading and a cassette handler are also included in a CMP apparatus.
As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrogenation reaction; the formation of hydrogen bonds between the wafer and the slurry; the formation of molecular bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface.
While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in controlling polishing rates at different locations on a wafer surface. Since the polishing rate applied to a wafer surface is generally proportional to the relative rotational velocity of the polishing pad, the polishing rate at a specific point on the wafer surface depends on the distance from the axis of rotation. In other words, the polishing rate obtained at the edge portion of the wafer that is closest to the rotational axis of the polishing pad is less than the polishing rate obtained at the opposite edge of the wafer. Even though this is compensated for by rotating the wafer surface during the polishing process such that a uniform average polishing rate can be obtained, the wafer surface, in general, is exposed to a variable polishing rate during the CMP process.
Recently, a chemical mechanical polishing method has been developed in which the polishing pad is not moved in a rotational manner but instead, in a linear manner. It is therefore named as a linear chemical mechanical polishing process, in which a polishing pad is moved in a linear manner in relation to a rotating wafer surface. The linear polishing method affords a more uniform polishing rate across a wafer surface throughout a planarization process for the removal of a film layer from the surface of a wafer. One added advantage of the linear CMP system is the simpler construction of the apparatus, and this not only reduces the cost of the apparatus but also reduces the floor space required in a clean room environment.
A typical conventional CMP apparatus 90 is shown in FIG. 1 and includes a base 100; polishing pads 210a, 210b, and 210c provided on the base 100; a head clean load/unload (HCLU) station 360 which includes a load cup 300 for the loading and unloading of wafers (not shown) onto and from, respectively, the polishing pads; and a head rotation unit 400 having multiple polishing pads 410a, 410b, 410c and 410d for holding and fixedly rotating the wafers on the polishing pads.
The three polishing pads 210a, 210b and 210c facilitate simultaneous processing of multiple wafers in a short time. Each of the polishing pads is mounted on a rotatable carousel (not shown). Pad conditioners 211a, 221b and 211c are typically provided on the base 100 and can be swept over the respective polishing pads for conditioning of the polishing pads. Slurry supply arms 212a, 212b and 212c are further provided on the base 100 for supplying slurry to the surfaces of the respective polishing pads.
The polishing heads 410a, 410b, 410c and 410d of the head rotation unit 400 are mounted on respective rotation shafts 420a, 420b, 420c, and 420d which are rotated by a driving mechanism (not shown) inside the frame 401 of the head rotation unit 400. The polishing heads hold respective wafers (not shown) and press the wafers against the top surfaces of the respective polishing pads 210a, 210b and 210c. In this manner, material layers are removed from the respective wafers. The head rotation unit 400 is supported on the base 100 by a rotary bearing 402 during the CMP process.
The load cup 300 includes a circular lift pedestal 310 on which the wafers are placed for loading of the wafers onto the polishing pads 210a, 210b and 210c, and unloading of the wafers from the polishing pads. Before each wafer is unloaded from the lift pedestal 310 onto the polishing pad or unloaded from the polishing pad back onto the lift pedestal 310, the lift pedestal 310 is extended upwardly from the load cup 300 by actuation of a pneumatic pedestal lift cylinder 320 beneath the base 100, as shown in FIG. 1A.
As further shown in FIG. 1A, the pedestal lift cylinder 320 is mounted on a cylinder mount element 350 beneath the base 100. A lift piston 330 is extendible from the cylinder 320 for selective raising and lowering of the lift pedestal 310 and a wafer (not shown) supported thereon for loading and unloading of the wafer on one of the polishing pads 210a, 210b and 210c. On the Mirra/Mesa CMP apparatus available from Applied Materials, Inc., of Santa Clara, Calif., a radial direction error-tolerance coupling 340 releasably connects the upper end of the lift piston 330 to the lift pedestal 310.
After prolonged use, the pedestal lift cylinder 320 becomes worn, and eventually, CDA (clean, dry air) used to effect the lifting and lowering actions of the lift piston 330 leaks from the cylinder 320. Consequently, the cylinder 320 becomes less efficient and must therefore be removed from the load cup 300 and replaced typically about every 6 months. However, this cylinder-removal procedure is cumbersome and time-consuming, as the load cup 300 must be removed from the base 100; the fastening screw which holds the pedestal 310 on the coupling 340 loosened; and the up/down sensors and other accessory equipment removed from the cylinder 320. Those steps are reversed for installation of a replacement cylinder 300 on the load cup 300. Furthermore, the load cup 300 is relatively heavy (about 20 kg), and thus, requires two personnel for safe handling during removal and replacement of the load cup 300 on the base 100. Accordingly, a new and improved, slide-type quick connect/disconnect coupling is needed for attaching a lift piston of a cup actuating cylinder to a lift pedestal of a load cup.
An object of the present invention is to provide a new and improved coupling suitable for attaching a pedestal lift cylinder to a lift pedestal of a load cup for a CMP apparatus.
Another object of the present invention is to provide a new and improved, slide-type coupling suitable for attaching a pedestal lift cylinder to a lift pedestal of a CMP load cup.
Still another object of the present invention is to provide a new and improved cylinder coupling which is suitable for substantially reducing the time required for replacing a pneumatic pedestal lift cylinder for a load cup on a CMP apparatus.
Yet another object of the present invention is to provide a new and improved cylinder coupling which is capable of substantially reducing the number of personnel required for replacement of a pedestal lift cylinder for a load cup on a CMP apparatus.
A still further object of the present invention is to provide a new and improved cylinder coupling including a T-shaped coupling bolt which is provided on a lift piston extendible from a pedestal lift cylinder and a coupling bracket which is provided on the bottom surface of a lift pedestal of a load cup for a CMP or other processing tool, which coupling bolt slidably engages the coupling bracket to provide a quick connect/disconnect attachment of the lift piston to the lift pedestal.
Yet another object of the present invention is to provide a new and improved cylinder coupling which is capable of facilitating quick connect/disconnect attachment between an actuating cylinder and an element to be displaced by the actuating cylinder in a variety of mechanical and industrial applications.